This invention relates generally to image scanning devices such as copiers, facsimile machines, and image scanners for computers, and more specifically to increasing the optical sampling rate through use of an optical waveguide.
Image scanners convert a visible image on a document or photograph, or an image in a transparent medium, into an electronic form suitable for copying, storing or processing by a computer. An image scanner may be a separate device or an image scanner may be a part of a copier, part of a facsimile machine, or part of a multipurpose device. Reflective image scanners typically have a controlled source of light, and light is reflected off the surface of a document, through an optics system, and onto an array of photosensitive devices. The photosensitive devices convert received light intensity into an electronic signal. Transparency image scanners pass light through a transparent image, for example a photographic positive slide, through an optics system, and then onto an array of photosensitive devices. For convenience, in the following discussion, reference may be made to the xe2x80x9cdocumentxe2x80x9d being scanned, but the invention is equally applicable to any image being scanned, whether an opaque document, a transparent film, or a scene with the imaging device focused at infinity.
Sensor arrays may be one-dimensional or two-dimensional. For convenience, in the following discussion, an optics system is used to focus a line on the document image (called a scan line) onto a one-dimensional sensor array, but the invention is equally applicable to two-dimensional sensor arrays.
A picture element (pixel) may be defined as an area on the image being scanned, or as an area on a photosensor array, or as a set of numbers in the data representing an image. For document scanners and transparent film scanners, a pixel is commonly defined as an area on the surface of the document being scanned. For example, for document and transparent film scanners, a common specification is xe2x80x9cpixels per inchxe2x80x9d (or mm) as measured on the surface of the document being scanned. Photosensor arrays typically have thousands of individual photosensitive elements. Each photosensitive element (or perhaps a set of photosensitive elements for color), in conjunction with the scanner optics, measures light intensity from an effective area on the document being scanned, thereby defining one pixel on the document being scanned. The optical sampling rate is the number of samples optically captured from one scan line divided by the length of the scan line. This number is often called xe2x80x9cresolution,xe2x80x9d but a more precise definition of resolution is the ability of a scanner to resolve detail, which includes many factors including the modulation transfer function of the optics and various noise factors.
Typically, for black-and-white or grayscale, there is a one-to-one correspondence between one pixel on the document being scanned, one sensor element, and one numerical intensity measurement. Typically, for color, at least three sensor elements are required to sense all the colors for one pixel on the original image, and at least three numerical intensity values are required to represent all the colors for one pixel on the original image.
There are two types of sensor arrays commonly used for image scanners. In a first type, the length of the one-dimensional sensor array is as long as the one-dimensional scan line on the image being scanned. These sensor arrays, called Contact Imaging Sensors (CIS), have one advantage in that relatively expensive reduction optics are not required (although optical waveguides or other focusing devices may be required). However, because of their relatively large size, overall arrays typically comprise an assembly of smaller arrays. Accurately assembling multiple smaller arrays adds cost.
In the second type of commonly used sensor arrays, the length of the one-dimensional sensor array is much smaller than the length of the one-dimensional scan line on the image being scanned. These small sensor arrays require an optics system to focus a line on the document image (called a scan line) onto the sensor array. See, for example, K. Douglas Gennetten and Michael J. Steinle, xe2x80x9cDesigning a Scanner with Color Vision,xe2x80x9d Hewlett-Packard Journal, August, 1993, pp 52-58. If a lens based optics system is used, increasing resolution typically increases the cost of the lens system because reduced aberrations and other improvements in the modulation transfer function of the optics are required.
Charge coupled devices (CCD""s) are frequently used for the second type of sensor arrays. Typically, CCD arrays comprise a single integrated circuit so that the expense of assembling multiple arrays is avoided. Increasing the optical sampling rate requires more pixels per scan line, and therefore, requires more CCD elements per scan line. Increasing the number of CCD elements typically results in more overall circuit area and longer arrays, and therefore higher cost. One approach to reducing the area and length of the overall arrays is to reduce the size of individual photosensitive elements. In general, regardless of the size of an individual photosensitive element, each photosensitive element receives, through an optics assembly, the light from a fixed pixel area on the image being scanned. That is, the total light received by a photosensitive element is determined by the size of the pixel on the image being scanned and ideally is independent of the size of the photosensitive element. Therefore, instead of sensitivity considerations, the minimum size of a photosensitive element is typically determined by integration circuit fabrication technology or the optics system (for example, fundamental diffraction limits). Typical CCD arrays for color scanners have active areas that are already close to a minimum practical size. Therefore, for linear arrays, increasing the optical sampling rate forces the overall length of the arrays to increase. For example, a typical CCD element for line image scanners is about 7 xcexcm by 7 xcexcm. A document with a width of 8.5 inches (216 mm) and an optical sampling rate of 1200 pixels per inch (47 pixels per mm) requires 10,200 photosensors per row. If each element is 7 xcexcm wide, the length of each linear array must be at least 71 mm long just for the sensor elements. In a typical array layout, additional length is needed for other electronics. For commonly used five inch (127 mm) diameter silicon wafers, the result may be that only one sensor assembly can be fabricated on each silicon wafer. This greatly increases the expense of sensor arrays. Alternatively, as for CIS modules, multiple sensor array segments may be assembled to provide an overall larger sensor, with the additional cost of assembling multiple segments.
There is a need for high scanning optical sampling rate without requiring a corresponding increase in the expense of the sensor array and without requiring expensive lens based optics.
A scanner in accordance with various example embodiments of the invention multiplexes light from multiple pixels on the document being scanned onto each sensor element. Instead of a one-to-one or many-to-one relationship between sensor elements and one pixel on the document, there is a many-to-one relationship between pixels on the document and one sensor element. In the various example embodiments, an optical waveguide is used to guide light from multiple pixels on the document being scanned onto a single photosensor element. Each sensor element measures the light from multiple pixels on the document, one at a time. That is, the optical waveguide has a higher optical sampling rate than the sensor array. Various methods are disclosed for selecting one original image pixel at a time for projection onto a single sensor element. If, for example, four image pixels are guided onto a single sensor element, then four separate scans are required to capture all the image pixels. Data from the four scans are then combined to form the overall data for a single scanned image. The resulting image has four times more true non-interpolated pixel data relative to the data that would be normally specified by the optical sampling rate of the sensor array. The method can be generalized so that N pixels on the image being scanned are guided onto M sensor elements. Superresolution image analysis techniques developed for reconstruction of one image from a set of lower resolution images may be applied to provide a diffraction-limited high-resolution image. The waveguide as described also provides an ability to reduce scanning time for lower resolution images.
In a first example embodiment, a rotating rod lens with a pattern of black and white areas is used to block/unblock optical waveguide array elements. In a second example embodiment, a thin mask is moved over the optical waveguide array elements. In a third example embodiment, a display array technology having electronically controllable transparent and opaque areas, for example a liquid crystal array, is used to block/unblock optical waveguide array elements.